Conventional wisdom has it that City Council Speaker Christine Quinn is the most fiscally responsible of all the prospective 2013 mayoral candidates.

That myth — carefully propagated by the speaker and her (erstwhile?) ally, Mayor Bloomberg — continues to be battered to bits, mostly by Quinn herself.

Certainly, Quinn is feeling triumphant today, after Manhattan Supreme Court Justice Judith Gische ruled in favor of her lawsuit against Bloomberg’s new homeless-shelter policy for single adults.

Gische blocked the proposed change in eligibility standards, ruling that City Hall failed to vet the policy before the public or the City Council, as required by law.

That’s not implausible; certainly it wouldn’t be the first time the administration fumbled the fine print.

Nevertheless, that doesn’t address Quinn’s motives in bringing the action.

In fact, she hailed the ruling as a “tremendous victory” against a policy that “would have needlessly put thousands of homeless New Yorkers on the streets.”

Which demonstrates, if nothing else, that the speaker is nostalgic for the bad old days when the city all but drafted fraudsters to live rent-free — courtesy of the taxpayers.

The new policy is simple — and eminently reasonable.

It requires single adults seeking a spot in a city homeless shelter to prove that they have no other housing options.

That’s the same rule that applies to families — who for years could simply claim homelessness, even if they weren’t, and then choose from an array of city-funded-and-furnished apartments.

As Bloomberg said yesterday: “Let the judges explain to the public why they think that you should just have a right to walk in and say, ‘Whether or not I need services, give it to me.’”

Add Christine Quinn to the list.

At least Gische didn’t invalidate the policy itself, ruling only on the procedure the city used to implement it.

Quinn, on the other hand, is leading the charge to scrap the whole thing.

Just as she wants to scrap finger-imaging — a time-tested anti-fraud reform — for food-stamp applicants because it’s “stigmatizing.” (Shouldn’t living off the sweat of another’s brow be stigmatizing?)

Just as she’s encouraging the council’s General Welfare Committee to create a new bureaucracy to guide people under 25 onto the welfare rolls — thus shifting the city away from its current emphasis on bringing them into the workforce.

Homeless shelters should be a last resort — not the first available option. That’s the only policy that makes sense even in good times — let alone when the city has to deal with an ailing fisc.

A mayor needs to make tough decisions that benefit the whole city, even if they’re unpopular with his or her political base.